Modelling the growth and water uptake function of plant root systems: a review

Abstract

Crop models have been intensively used as a tool to analyse the performance of cropping systems under variable climate in terms of productivity, profitability, and off-site impact. The importance of modelling the function of plant roots in water and nutrient uptake from the soil is becoming increasing clear with the expanding application areas of crop models. This paper reviews the approaches and assumptions used in growth and uptake modelling of plant roots, and how the responses of plant root system to internal and external factors are captured in the widely used crop models. Most modelling approaches are based on one of the following assumptions: (i) that plant roots are uniformly distributed in homogenous soil layers and all roots have the same ability for uptake, or (ii) that plant root length is always sufficient for resource uptake in rooted soil layers. In structured soils, an overestimation of water uptake is likely to be expected. Further studies on root growth, distribution, and function in structured soils will require quantification of soil structures and root distribution patterns; and for non-uniformly distributed plant populations, spatial distribution of plant roots and non-uniform uptake need to be modelled. Root architecture modelling may help to address such issues. However, in order for the model to be useful at the field production level, simplified approaches that require easily measurable inputs need to be developed.Some examples are given. The oversimplification of root response to soil drying and hardness is likely to lead to overestimation of root growth and water uptake in dense soils. A soil strength factor needs to be incorporated so that the improved model can help evaluate the effect of subsoil compaction on production and resource use. Responses of root growth and uptake to soil salinity, boron toxicity, and extreme pH need to be further investigated if models are to be used for evaluation of crop performance in such environments. Effect of waterlogging also needs to be added for use of the model on heavy clay soils under irrigation or concentrated rainfall. There is an urgent need for joint efforts of crop physiologists, agronomists, breeders, and soil scientists to integrate interdisciplinary knowledge and to collect data that better describe the crop root system and its growth and uptake ability, to quantify plant process level responses, and for better soil quantification. Such knowledge and data are essential for improvement of model performance and successful applications. Lucerne Oat Tap root Branches Crown roots Seminal roots Fig. 1. Crop root system. Lucerne has one tap root and branches, whereas oat has seminal and crown roots (modified from Weaver 1926).Growth and water uptake of plant roots

Simple physics-based models of compensatory plant water uptake: concepts and eco-hydrological consequences

“…less than 10 % of the total root length), because of poor hydraulic contact of roots with soil and its heterogeneous distribution in undisturbed structured soil (e.g. Wang and Smith, 2004;White and Kirkegaard, 2010). If this is the case, the difference between Eqs.…”

“…Sub-models of root water uptake are therefore sensitive and important building blocks in ecohydrological models that simulate terrestrial water, energy and carbon balances to support, for example, crop growth or global climate models (Wang and Smith, 2004;Seneviratne et al, 2010). One especially important and difficult task for root water uptake models is to reflect the dynamic response of plant uptake to water stress, in which uptake N. J. Jarvis: Simple physics-based models of compensatory plant water uptake increases from sparsely rooted but well-watered parts of the root zone to compensate for stress in other parts.…”

“…One important unresolved challenge is to develop root water uptake models which are both relatively parsimonious with respect to data requirements and can also capture the complexity of the bio-physical response mechanisms underlying compensatory uptake in a realistic way (Doussin, 2003;Wang and Smith, 2004). "Root architecture" models linked to soil water flow models (e.g.…”

Root water uptake and profile soil water as affected by vertical root distribution

“…However, our ability to predicting the complex processes of belowground water uptake is still limited. This is because, on the one hand, many existing models are still not suitable for the scenarios that root distribution deviates from the regular exponential pattern, although different levels of detail and consequently different types of plant root information have been used in the model parameterizations (Smucker and Aiken 1992;Calmon et al 1999;Wang and Smith 2004). On the other hand, many studies based on the simplified root-soil-water system have not provided a knowledge foundation that is sufficient for our understanding on the complex process of root water uptake.…”

“…It has been considered as a key factor in optimization of modern agricultural practices involving irrigation and fertilization (Clothier and Green 1994;Moroke et al 2005). Studies indicate that root water uptake is one of the consequences of ecophysiological interactions between belowground and aboveground processes (Zhuang et al 2000), and is largely affected by root system distribution, soil hydraulic properties, and climate conditions (Jackson et al 2000;Feddes et al 2001;Zhuang et al 2001a;Wang and Smith 2004). In most soil profiles, root density usually decreases exponentially with soil depth, but soil compaction and layered soil structure can considerably modify this regular root distribution (Atwell 1993;Tardieu 1994;Passioura 2002;Araki and Iijima 2005).…”